Melomys is a genus of rodents from Oceania, including several similar-looking species nearby in parts of Australia, Indonesia and Papua New Guinea. But the Bramble Cay melomys was a distinct species with an island of its own, and the only mammal native to the Great Barrier Reef. Unlike the invasive ship rats known for overwhelming islands elsewhere, it was already on Bramble Cay when Europeans arrived in 1845. Early in the 20th century, scientists formally titled it Melomys rubicola.

As recently as 1978, Bramble Cay supported up to several hundred of these rodents, a type known as mosaic-tailed rats. A 1998 survey found only 42, leading to a total population estimate of 93. Follow-ups revealed just 10 rats in 2002 and 12 in 2004, including the last one ever captured by scientists. A fisherman reported one final sighting in 2009, then the species seemed to disappear.

The Bramble Cay melomys, pictured here in 2002, hasn't been seen since 2009. (Photo: Ian Bell/University of Queensland)

Hoping to find a few survivors, researchers from Queensland University conducted new surveys of Bramble Cay in 2014. Their effort involved 900 small-mammal "trap nights" (one trap set for one night) and 600 camera-trap nights, plus active daytime searches of the island, which is smaller than Madison Square Garden.

This week, after a long review of their data and of other studies, the researchers announced their conclusion: The Bramble Cay melomys is now extinct in its only known habitat, and "probably represents the first recorded mammalian extinction due to anthropogenic climate change."

The main reason for the species' downfall, they explain, was almost certainly ocean flooding in the past decade, "very likely on multiple occasions." The cay's highest point is just 3 meters (9.8 feet) above sea level, and inundation by seawater can kill the plants that provided the Bramble Cay melomys with food and shelter.

Bramble Cay, aka Maizab Kaur, is on the northern tip of the Great Barrier Reef. (Map: University of Queensland)

Overall, Earth's sea level rose by 19 centimeters (7.4 inches) from 1901 to 2010, a rate unseen in 6,000 years. The average rise over that period was 1.7 millimeters per year, the report notes, and about 3.2 mm per year from 1993 to 2014, an increase driven by human-induced climate change via melting glaciers and thermal expansion of seawater. At this rate, the ocean could rise 1.3 meters (4.3 feet) within 80 years.

But there are regional variations in sea-level rise, and it has been extreme around Northern Australia, they add. "Tidal gauge and satellite data from the Torres Strait and Papua New Guinea indicate mean sea level has risen 6 mm per year between 1993 and 2010 for the region, a figure that is twice the global average," the report states. "The Torres Strait islands are particularly vulnerable to rising sea levels and low-lying communities here are already subjected to regular inundation by the sea, with spring tides each year causing increasing extents of flooding and erosion."

The amount of land above high tide at Bramble Cay shrunk from 4 hectares (9.9 acres) in 1998 to just 2.5 hectares (6.2 acres) in 2014, and that wasn't even the worst news for local rodents. The island also lost 97 percent of its vegetation cover in 10 years, from 2.2 hectares (5.4 acres) in 2004 to 0.065 hectares (0.2 acres) in 2014.

Researchers set these rodents traps in the cay's vegetation, but to no avail. (Photo: Ian Gynther/University of Queensland)

That gave the Bramble Cay melomys little chance of survival, leaving the entire species vulnerable to a single storm or flood. The researchers say it's still possible that an undiscovered population persists off the island, maybe on the Papua New Guinea, but that's a long shot. This creature is most likely gone forever, and while it's just one species among millions, it's hardly an isolated case.

Earth is in the midst of a mass extinction event, fueled by climate change as well as other human activities like deforestation, pollution and poaching. The planet had at least five extinction events before now, but this is the first in human history — and the first with human help. Earth's entire vertebrate population has fallen 52 percent in the last 45 years alone, and the threat of extinction still looms for many — including an estimated 26 percent of all mammal species. A 2015 study estimated that one of every six species is at risk of extinction due to climate change.

According to another recent study, "the average rate of vertebrate species loss over the last century is up to 114 times higher than the background rate." The authors pegged that background rate at two mammal extinctions per 10,000 species per 100 years (2 E/MSY), which is double the baseline used in many studies.

"Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear," the study's authors wrote. "These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way."

When a rat goes overboard, it's usually a good idea to pay attention. Even if you don't care about rats themselves, it might be a sign the ship is sinking.